It is generally recognized that the use of fossil fuel such as coal and petroleum as energy sources invites global warming by resultant carbon dioxide. The use of atomic energy is accompanied by the risk of contamination by radioactive rays. Currently under various discussions on the environmental issues, dependence upon these kinds of energy is undesirable.
On the other hand, solar cells, which are photoelectric conversion devices for converting sunlight to electric energy, use sunlight as their energy resources, and they produce only a small adverse effect to the global environment. Therefore, wider promulgation of solar cells is anticipated.
Although there are various materials of solar cells, a number of solar cells using silicon are commercially available. These solar cells are roughly classified to crystalline silicon solar cells using single crystal or polycrystal and amorphous silicon solar cells. In conventional solar cells, single crystal silicon or polycrystal silicon, i.e. crystalline silicon, has been used often.
In crystalline silicon solar cells, photoelectric conversion efficiency, which represents the performance of converting light (sun) energy to electrical energy, is higher than that of amorphous silicon solar cells. However, since crystalline silicon solar cells need much energy and time for crystal growth, they are disadvantageous from the viewpoint of their costs.
Amorphous silicon solar cells are advantageous in higher light absorption, wider selectable range of substrates and easier enlargement of the scale. However, photoelectric conversion efficiency of amorphous silicon solar cells is lower than that of crystalline silicon solar cells. Furthermore, although amorphous silicon solar cells are higher in productivity than crystalline silicon solar cells, they need an evacuation process for the manufacture. Therefore, the burden related to facilities for fabrication of crystalline silicon solar cells is still heavy.
On the other hand, there have been long researches in solar cells using organic materials toward more cost reduction of solar cells. However, they exhibit as low photoelectric conversion coefficient as 1%, and involve difficulties in durability.
Under the circumstances, an inexpensive solar cell using dye-sensitized porous semiconductor particles was introduced in Nature 353, p. 737, 1991 (Literature 1). This is a wet solar cell, i.e. an electrochemical photocell whose photo electrode is a titanium oxide porous film spectrally sensitized by using ruthenium complex as a sensitizing dye. Advantages of this solar cell are: permitting the use of inexpensive oxide semiconductors such as titanium oxide; wide light absorption of the sensitizing dye over the visible wavelength up to 800 nm; and high quantum efficiency of the photoelectric conversion enough to realize a high energy conversion efficiency. Moreover, because the solar cell does not need a process in a vacuum for its manufacture, it does not require bulky facilities or equipment.
This solar cell, however, requires the use of a substrate resistible to the baking temperature as high as approximately 500° C. in the high-temperature burning process for making its porous semiconductor electrode, and here is the problem of reducing the selectable range of substrates. In this connection, a number of researches have been reported regarding methods of making semiconductor electrodes by baking at temperatures lower than 300° C. as well as such methods by dry film deposition or wet electrolytic deposition without relying on the baking process. However, semiconductor electrodes made by these methods are not sufficient in durability, and photoelectric conversion efficiency of solar cells currently remains lower than several %.
Under the circumstances, A. Hagfeldt et al. disclosed a process capable of making a semiconductor electrode at room temperatures by coating a paste of titanium oxide nanoparticles containing a binder on a substrate and pressing the paste to bond the semiconductor nanoparticles onto the substrate (Journal of Photochemistry and Photobiology A: Chemistry, 145, p. 107, 2001 (Literature 2)).
According to Literature 2, photoelectric conversion efficiency of a dye-sensitized solar cell having the semiconductor electrode made at room temperature reaches 4 to 5%. However, the photoelectric conversion efficiency is still lower than that of a solar cell having a semiconductor electrode made by the baking process. In addition, although this method of making the semiconductor electrode at room temperatures makes it possible to use even a plastic substrate having low heat resistance as a support member of a transparent electrode, the semiconductor nanoparticle layer formed on the plastic substrate by pressure bonding is low in adhesiveness to the substrate and flexibility, and therefore unreliable in durability against bending or expansion and contraction. In addition, since ethyl cellulose used in Literature 2 as a binding agent is soluble to alcohol and organic solvents, it dissolves into the dye solution and electrolytic solution used for the dying with the dye, and invites serious characteristic deterioration with time.
It is therefore an object of the invention is to provide a photoelectric conversion device that is enhanced in adhesiveness to the substrate of the semiconductor electrode made of semiconductor nanoparticles and in flexibility of the semiconductor electrode, also enhanced in durability against bending or expansion and extraction, and excellent in photoelectric conversion property, as well as a manufacturing method of the device.
More generally, the object of the invention is to provide an electronic apparatus that is enhanced in adhesiveness of a substrate of a semiconductor electrode made of semiconductor nanoparticles and in flexibility of the semiconductor electrode, also enhanced in durability against bending or expansion and contraction, and excellent in property, as well as a manufacturing method of the device.